Colon cancer is one of the most frequent human malignancies, with 150,000 new cases and 60,000 deaths per year in the United States. Some of the molecular changes involved in the development of colon tumors are known, and in about 20% of the cases the genetic basis for colon tumor susceptibility has been identified, either as a mutation in the Apc locus or as a mutation in a locus encoding a protein involved in DNA synthesis/repair. The genetic basis for susceptibility to sporadic rumors, about 80% of cases, is unknown and is probably complex. We have been working with a mouse model for susceptibility to carcinogen-induced colon cancer, first using the carcinogen 1,2-dimethylhydrazine dihydrochloride (DMH) and later its metabolite azoxymethane, (AOM), which is more stable and requires fewer injections. Strain ICR/Ha is highly sensitive to DMH-induced colon tumors, while strain C57BL/6Ha is resistant. The (ICRxC57BL/6) F1 hybrid was backcrossed to C57BL/6Ha, and 122 progeny were treated with DMH. Colon tumors were detected in some of these progeny, which were typed for genetic loci on all chromosomes. QTL analysis using the number of tumors per animal suggested that four chromosomal regions, on Chrs 5, 12, 13 and 14, contained genes with allelic differences that affected the number of carcinogen-induced colon tumors. A set of congenic strains have been constructed that separately place the sensitive alleles at each of the four chromosomal regions on the background of the resistant strain. Animals from generation N5F2 of each of these strains were treated with DMH and colon tumors were found in most animals from all four congenic lines. Subcongenic lines have been generated for all chromosomes and two sets (Chrs 12 and 14) have been tested and are sensitive to AOM. We now propose four Specific Aims. In the first aim we plan to further refine the mapping of each gene by decreasing the length of the congenic region. In the second aim we plan to refine our protocol for use of AOM to determine the best injection schedule so that all ICR/Ha mice develop tumors. Following this, animals from the subcongenic strains will be tested to identify the smallest subcongenic interval containing the gene determining sensitivity. In the third aim we will address the question of what genetic changes occur in the tumors themselves, and whether there is specificity such that tumors arising from any one of the congenic strains show a consistent profile of changes.